There are 3 variants of Patt 23 that use the standard lens tube which has two lens positions: Patt 23 uses a 3 1/2" x 5" lens in the front, and a spill ring at the back. Patt 23W uses a 3 1/2" x 5" lens in the front, and another 3 1/2" x 5" lens at the back. Patt 23F uses a 3 1/2" x 4" Fresnel lens at the front, and a spill ring at the back. I used to work occasionally for an LD that had a box of spre lenses, it wasn't uncommon to have to add the second lens into the lens tube at the top of a Tallescope to gain a little extra spread! I can't think of a common fresnel with a 3 1/2" lens, minuette and acclaim are both 4 1/2" I think, and they are the smallest generlly avilable.